Carbon fiber reinforced plastic (“CFRP”) is widely used to provide structural members having a high strength to weight ratio. Carbon fiber reinforced plastics are fabricated from polymeric composites including thermosetting or thermoplastic polymer (“resin”) matrix within which are embedded carbon fibers. CFRP structural members may be formed by layers of fiber reinforced sheets and then bonding the sheets to form a composite member. These composites are widely used for aircraft structures, for example, the fuselage, wings, and other components of the aircraft's outer shell. These composites may be reinforced by thin metal sheets between layers of the composite member to enhance structural integrity of the member.
CFRP is also used to form bending and stiffening members for structures such as aircraft. Bending and stiffening members are typically elongate members formed to have a cross section that resists bending about axes lying in the cross section. I sections, T sections and C sections are examples of such members. Forming such members of laminated CFRP sheets provides a member having advantageous strength along the member for bending and stiffening. However, applying loads to such members at connections such as to other members or components presents difficulties.
CFRP members provide significant strength along the directions lying along the sheets that comprise the member. However the strength of such members through their thickness, a direction from sheet to sheet, is primarily the strength of the bonds between sheets and the strength of the plastic matrix of sheets that support fibers. Those strengths are orders of magnitude less than the strength of the fiber reinforced sheet in directions along which fibers extend. This low through thickness strength presents problems at connections to bending and stiffening members. Such connections can be by conventional connectors such as bolts or rivets extending through an opening that extends in a through thickness direction of the CFRP member. Loads applied to the CFRP member by such connectors can damage the CFRP member by causing delamination of sheets forming the member or damage to the matrix within which the fibers are embedded. Such damage typically occurs in regions where the load path changes direction from one plane to another plane that is inclined to said one plane (e.g. the radius detail when transitioning from a flange to a web).
FIG. 1 shows known reinforcements for connection to an I section CFRP member 10. The member 10 is formed by two separated generally parallel flanges 14 and 16 that are connected by a web 12 extending between and generally perpendicular to the flanges 14 and 16. FIG. 1 illustrates two connection reinforcements, radius block 22 and angle fitting 26. Radius block 22 and angle fitting 26 strengthen connections to the member 10 through the flange 16 from a surface 17 that faces oppositely from the flange 14. The connectors 24 and 28 extend through the flange 16 to connect to a component or member adjacent to the surface 17.
The connector 24 includes a section that overlies the radius block 22 and another section that extends through the radius block 22 and the flange 16. The radius block 22 is positioned adjacent to a surface of the flange 16 opposite the surface 17 to distribute any load applied through the connector 24 to the flange 16. The connectors 28 similarly extend through the angle fitting 26 and the flange 16. In addition, the angle fitting 26 extends adjacent to the web 12 and connectors 30 extend through the angle fitting 26 and the web 12 to additionally reinforce the flange 16 to web 12 integrity of the member 10 and to transfer load from the connectors 28 to the web 12.
Radius blocks, angle fittings and similar connection reinforcements for applying loadings to CFRP members provides needed connection strength for CFRP members. Such connection reinforcements are additional parts that must be positioned and secured during assembly of structures that include connections to bending or stiffening members such as member 10. These additional parts and the effort required for their assembly into a structure adds cost to structures that include bending or stiffening members such as member 10 and adds effort to assembly of those structures.